Researches On MEMS Thermal Wind Sensors For Low Power Applications

The accurate measurements of wind speed and direction are essential to various aspects of daily life and industrial production.Thus,wind sensors are widely used in different fields such as agricultural production,transportation,and equipment manufacturing,etc.In recent years,with the development of microelectromechanical systems(MEMS)technology,a variety of new MEMS wind sensors are emerging.Among them,MEMS thermal wind sensors have drawn a great attention because of their advantages of non movable structure,high initial sensitivity,low cost and so on.However,thermal wind sensors also have a main drawback.They consume much more power than their counterparts such as mechanical and ultrasonic anemometers because the sensor chips need to be heated to above ambient temperature.This limits their application in mobile meteorological measuring equipment and other low-power occasions.Therefore,in this dissertation the theoretical model,structure design,fabrication process,chip packaging and other aspects of thermal wind sensors are studied comprehensively and thoroughly,for the purpose of developing a low power,high performance,and robust MEMS thermal wind sensor.The main work of this dissertation includes:(1)A novel low power MEMS thermal wind sensor with its backsurface for wind sensing is proposed.The sensor was realized on one side of a silicon-in-glass(SIG)substrate.Vertical silicon vias in the substrate ensure good thermal contact between the sensor and the airflow,while the glass effectively isolates the heaters from the thermistors.The substrate was fabricated by using a glass reflow process and a lift-off process is followed to realize the heating and sensing elements.The sensor substrate was directly used as a packaging board,which reduces the deviation caused by chip packaging process and has a low manufacturing cost.Finally,the sensor was tested and calibrated in a wind tunnel.At a constant heating power of 24.5 mW,measurement results show that the sensor can detect airflow speeds of up to 25 m/s,with an accuracy of 0.1 m/s at low speeds and 0.5 m/s at high speeds.Airflow direction can be determined in a 360° range with an accuracy of ±6°.(2)The effect of packaging asymmetry on the performance of 2D MEMS thermal wind sensors with different heating geometries is investigated quantitatively for the first time.In the experiment,the wind sensor was operated by using different heating geometries,which are respectively annular heating geometry,square heating geometry and hybrid heating geometry.The experimental results show that the effect of packaging errors on the output signal of the sensor is mainly reflected in the shift of the output voltage amplitude and the phase shift of the wind direction angle.Due to the thermal asymmetry introduced by packaging errors,the accuracy of wind speed measurement will change with the wind direction,and the wind direction measurement results will also deviate from the actual values.It also can be found that the thermal wind sensor with annular heater geometry is less susceptible to package errors.(3)A novel and low cost method to improve the sensitivity of a low power 2D MEMS thermal wind sensor is proposed.By using HF wet etching,the thickness of the glass substrate decreases so that the sensor's thermal vias become more exposed to the wind.As a result,the conductive heat transfer is weakened and the convective heat transfer is enhanced in sensor operation.The measurement results show that after wet etching for 7 min and 14 min,the measured sensitivities of the sensor at 5 m/s have been improved by 27.7%and 112.5%,respectively.Measurement results also show that the improved sensitivity of the sensor chip can provide a more accurate measurement in wind speed but has little effect on the wind direction measurement.Instead,the accuracy of wind direction measurement is mainly related to the structural and thermal symmetries of the wind sensor.(4)A new type of packaging method for MEMS thermal wind sensor is developed.The low resistivity silicon vias embedded in the glass substrate are used to realize the electrical path between the sensor and the circuit implementation,which improves the reliability of the sensor signal extraction,and provides the possibility of the three-dimensional heterogeneit integration between the sensor and the circuit chip.The sensor was successfully prepared and packaged.The test results show that the total power consumption of the sensor is 14.5 mW,and the wind speed measurement range is 0-33 m/s,with an accuracy of 0.5 m/s at low speeds and 5%FS at high speeds.The sensor's sensitivity at 5m/s is about 20.2 mV/(m/s).The maximum wind direction error is less than 5°,whereas the average error is about 2°.(5)The effect of ambient temperature on the output signal of MEMS thermal wind sensor is studied.The analytical model of the sensor including its package structure was established firstly,which provides a quantitative relationship between the sensor's output signal and its structure parameters,material properties and heating powers.Then the voltage output of the packaged wind sensor under different temperature conditions was measured.The test results show that the output voltage signal of the wind sensor decreased with the increase of ambient temperature.The absolute value of the decrease is small at low wind speeds,whereas that is large at high wind speeds.However,the relative proportion of the decreased value to the output signal at a constant wind speed is almost the same for both conditions.With a temperature increase of IK,the output voltage of the sensor is decreased by about 0.75%.Based on the analytical model and the finite element simulation,it is found the temperature drift of the sensor output signal is mainly casued by the temperature drift of thermal properties of air,thermal conductivity of the sensor chip and thermal conductivity of the package structure.In a word,this dissertation proposes two kinds of low-power MEMS thermal wind sensor based on glass reflow proces.The design theory,structural optimization,fabrication process and chip packaging of these two sensors have been studied in-depth.The experimental results show that the proposed thermal wind senosrs have low power consumption,high sensitivity,wide measurement range and enough mechanical strength.Such characteristics make them suitable for the mobile weather monitoring equipment and other low-power applications.